Do not strip trailing whitespace in Git diffs

Problem: Editorconfig was configured to strip trailing whitespace in all
file types. This is generally a good thing, but when interactive
rebasing with Git and editing the contents of a commit, trailing
whitespace is significant on blank lines and must be preserved in order
for edits to be applied cleanly.

Solution: Update Editorconfig to exclude *.diff from the strip
whitespace rule, as interactive rebasing edits are done in a temporary
file with a '.diff' suffix.

.editorconfig

@@ -9,6 +9,9 @@ charset = utf-8
 trim_trailing_whitespace = true
 insert_final_newline = true
 
+[*.diff]
+trim_trailing_whitespace = false
+
 [Makefile]
 indent_style = tab